1. Field of the Invention
The present invention relates to a core type magnetron, and more specifically relates to such a magnetron wherein the permanent magnet of which has been improved.
2. Description of the Prior Art
A core type magnetron typically comprises a permanent magnet built inside a vacuum envelope which also encloses a cathode and anode vanes such that a magnetic field is applied to an operating space surrounding the cathode. Typically the core type magnetron has various advantages, as compared with a shell type magnetron wherein a permanent magnet is provided outside the vacuum envelope, since in the core type magnetron, the magnetic circuit is shorter, the permanent magnet is small in size and the vacuum envelope is formed of a magnetic material such as iron. Therefore, the vacuum envelope per se is used as a magnetic path.
In general, electronic tubes, including core type magnetrons, suffer from deterioration of performance, partly because of emission of undesired gasses from the structure of such electronic tubes. Particularly in magnetrons, emitted undesired gasses such as oxygen within the vacuum envelope degrade the surface condition of the cathode to decrease the rate of emission of electrons from the cathode and thereby cause the performance to be adversely affected.
Heretofore it has been proposed that proper countermeasures be taken to the structure components such as a cathode, anode vanes and the like during the manufacture of the magnetrons. However, in case of a core type magnetron, no countermeasure has been proposed for suppressing emission of undesired gasses from the permanent magnet located inside the vacuum envelope of the magnetron. In view of the fact that in a core type magnetron the permanent magnet occupies a comparatively large volume of the vacuum envelope and in addition the permanent magnet is heated to a high temperature during operation of the magnetron, the emission of gasses from the outer surface of the permanent magnet is a serious problem.
For the purpose of preventing emission of undesired gasses from the outer surface of the permanent magnet within the vacuum envelope, one might simply think of enclosing the permanent magnet with a given material such as a copper plate. However, this approach makes the structure of the magnetron complicated to manufacture and also requires the size of the vacuum envelope to be larger.